This invention relates generally to air filtration apparatus, and in particular to a side access filter assembly incorporating multiple filter elements.
There are a number of indoor air quality applications in which clean conditioned air must be supplied into an enclosed living space or work space that is exposed to hazardous airborne particulates. In the typical application, conditioned air is cleaned by one or more stages of air filters which receive fresh outdoor air along with return room air and are designed to absorb or capture undesirable chemical vapors and/or airborne particulate materials.
Conventional air filters include chemical filters with an impregnated activated filter media; high efficiency bag filters; high efficiency particulate arrestance (HEPA) filters; spun fiberglass and paper media; and synthetic filter media. A high efficiency bag filter is sometimes used as a secondary stage in combination with a pre-filter stage for capturing respirable (4.0 microns to 0.3 micron) particles. High efficiency filter arrangements are used in surgical facilities, medical wards, clean rooms, computer rooms, laboratories and other commercial applications where maintenance of clean and particle-free air is essential.
A widespread problem in the use of high efficiency air filters is bypass leakage around the filter element casing frames. Typically, multiple filter elements are arranged and supported in a rectangular array, sometimes stacked vertically in two or more rows across the air flow passage. Unsealed air gaps between adjacent filter elements and unsealed air gaps between the filter elements and the filter housing, which may be as large as 0.125 inch to 0.625 inch wide, present a path of least resistance in which a significant amount of unfiltered air leaks around and bypasses the filter elements and thus substantially reduces system efficiency. Consequently, a high efficiency bag filter designed to trap respirable particles can be compromised by an unsealed air gap which could be as much as 13,500 microns wide. Because air flow always follows the path of least resistance, more unfiltered air flows through such gaps as the air flow rate increases and the filter becomes saturated with particulates.
Various gaskets and seals have been employed or proposed for forming a positive seal between the filter elements and the surrounding air flow housing. Some conventional seal assemblies do not provide an adequate seal, and others interfere with the inspection and/or replacement of the filter elements. For example, open cell foam pads are installed where air gaps are likely to occur. Over time, the foam pads deteriorate, shrink and open a bypass air gap. Other arrangements use oversized filter housings and frames to provide flat surfaces for engaging multiple seal elements which create more by-pass leakage gaps as they deteriorate, thus increasing the maintenance burden. Since most airborne contaminates are not visible and are difficult to detect, the indoor air quality of critical clean rooms, hospital areas and delicate production processes may be compromised or placed at risk.
Conventional air filter elements are typically mounted on filter guide tracks that support the filter elements in an operative service position across the air flow passage. Conventional guide tracks are slightly oversized by as much as 0.54 inch to allow the filter elements to slide smoothly in and out. This oversizing creates a gap along the track and at opposite ends of the track through which contaminate-laden air can completely bypass the filter elements. A typical method for sealing track-mounted filter elements is by a strip of felt or nylon pile gasket material. The felt or nylon strip is packed in the filter tracks to fill the voids between the filter frame and the filter track channels.
Such packing materials typically do not completely fill the gaps between the filter tracks and the filter casing, or between the corners of the filters and the surrounding housing. Moreover, no provision is made for sealing the abutment gaps between adjacent filter elements that are loaded on the same filter track. A further limitation on the use of gasket packing material is that such sealing strips become compacted and deformed after repeated insertion and removal of the air filter elements. Upon compaction, the felt or nylon pile gasket becomes separated from the filter frame, thus opening a leakage gap.
Another seal assembly which is used in combination with track-mounted, multiple filter elements includes a foam gasket strip that is clamped against the casing face of each filter element. The clamp must be manually tightened to compress the foam gasket material between the filter casing face and the air flow housing. Such manual clamping arrangements are difficult to implement for those filter elements that lie beyond or near the limit of the operator's reach.
Accordingly, there is a continuing interest in providing an effective seal between one or more air filter element and the supporting track structure within a side access filter housing, which will permit the filter elements to be inserted into and retrieved from the air flow filter housing without damaging the seal elements, and which reliably produces a positive seal upon full insertion of the filter elements, without requiring manual clamping or other manual adjustment.